mesa: fix _mesa_max_texture_levels for GL_TEXTURE_EXTERNAL_OES

This helps fix:
    piglit/bin/ext_image_dma_buf_import-sample_yuv -fmt=NV12 -auto

Fixes: d88f3392ff

Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/main/teximage.c

@@ -530,7 +530,7 @@ _mesa_max_texture_levels(const struct gl_context *ctx, GLenum target)
          && ctx->Extensions.ARB_texture_multisample
          ? 1 : 0;
    case GL_TEXTURE_EXTERNAL_OES:
-      /* fall-through */
+      return _mesa_has_OES_EGL_image_external(ctx) ? 1 : 0;
    default:
       return 0; /* bad target */
    }